Electroluminescent quenching of a photoconductor through a substrate



United States Patent 3,173,014 ELECTROLUMINESCENT QUENCHING OF A PHOTOCONDUCTOR TOUGH A SUB- STRATE Gunther E. Fenner, Schenectady, N.Y., assignor to General Electric Company, a corporation of New York Filed June 30, 1961, Ser. No. 121,101 4 Claims. ((31. 250-213} This invention relates to electroluminescent circuits, devices and apparatus of the type utilizing photoconductor and electroluminescent elements in combination and in particular to such apparatus particularly adapted for detecting changes in the light level from a uniform background.

For a great many applications it is required to provide for the high speed and reliable detection of any change in the light level from a uniform background. A photoconductor element disposed in non-light exchange relationship with an electroluminescent element and electrically in parallel circuit therewith may be utilized for an application of this type wherein decrease in the light level from a predetermined steady state value causes a detectable output from the electroluminescent element. Because of the very large nonlinear-ides in the response of both of the elements in such a combination, analog signals can very effectively be converted into digital signals.

A serious limitation of such an arrangement, however, is the response time. The response time may be defined, for example, as the time required for the electrolumines cent element to read full output after the incident light level has fallen below a predetermined threshold value. In apparatus of this type, this time depends primarily upon the decay time of the photoconductor element by recombination of free carriers. Thus, it is a constant of the photoconductive material itself. Known prior art types of apparatus for detecting changes in light level from a uniform background have not been entirely satisfactory particularly for a great many applications which require very high response speeds and good sensitivity.

It is an object of this invention, therefore, to provide electroluminescent apparatus, devices and circuit arrangements particularly adapted for detecting changes in light level from a uniform background which substantially overcome one or more of the disadvantages of the prior art and which have greatly increased sensitivity and response speed.

i It is another object of this invention to provide apparatus utilizing a combination of a photoconductor and an electroluminescent means for detecting changes in light level from a uniform background having increased response speed and sensitivity by applying an appropriate photoconductivity quenching signal to the photoconductor element at the time of decreased incident light from the uniform background.

It is yet another object of this invention to provide a high speed electroluminescent-photoconductor circuit for providing analog to digital signal conversion.

Briefly stated, in accordance with one aspect of this invention, apparatus for detecting changes in the light level from a uniform background comprises a photoconductor element, which exhibits quenching of its photoconductivity when subjected to irradiation of predetermined photon energy, electrically connected in parallel circuit relationship with an electroluminescent means. Means are provided for connecting an appropriate voltage across the circuit. Means are further provided for applying an optical quenching signal to the photoconductor element at the time of decrease of incident light thereon from the uniform background. The quenching signal so applied decreases the response time of the apparatus by increas- 3,173,914 Patented Mar. 9, 1965 ing the rate of decay of the photocurrent in the photoconduotor element.

The novel features which are believed characteristic of this invention are set forth with particularity in the appended claims. My invention itself, however, both as to its organization and method of operation together with further objects and advantages thereof may best be understood by reference to the following description taken in conjunction with the accompanying drawing in which:

FIG. 1 illustrates the time dependence of the photoconduction of a sample of photoconductive material in response to radiation by photons of various energies,

FIG. 2 is a schematic circuit diagram of an electroluminescent switching circuit having increased response speed and constructed in accordance with one embodiment of this invention,

FIG. 3 is a diagrammatic sectional view of an electroluminescent device in accordance with one embodiment of this invention,

FIG. 4 is a graphical representation of the emission characteristic of a typical electroluminescent material suitable for use in the practice of this invention; and,

FIG. 5 is a diagrammatic sectional view of an electroluminescent device in accordance with another embodiment of this invention.

The present invention provides a novel electrical apparatus which makes use of the well-known phenomena of electroluminescence, photoconductivity, and quenching of photoconductivity. For example, it is well-known in the art that luminescence may be excited in certain materials by the action of electric fields. This phenomenon is known as electroluminescence and the materials as electroluminescent phosphors. Materials are known which exhibit apparently continuous luminescence when an alternating current voltage is suitable applied thereacross. Similarly, other materials are known which exhibit such apparently continuous luminescence when a direct current voltage is applied which is sufficient to cause a certain current flow therein. Although the above described phenomena are not fully understood, it appears to be generally agreed that the electroluminescence results from a redistribution of electrons in the crystal structure of the materials and the consequent emission of light therefrom.

It is also well-known that certain materials possess the property of varying their electrical conductivity under the influence of light. Such materials are referred to as photo-conductive materials and the phenomenon as photoconductivity.

Studies in the prior art have further shown that photoconductiv-ity in photoconductive materials can be quenched by subjecting a sample to an. appropriate infrared irradiation. For example, it has been found that the photoconductivity of a particular photoconductive material coud be quenched or enhanced depending upon the photon energy (hr) with which it is irradiated, where h is Planks constant and z/ is the photonfrequency. This quenching of photoconductivity for a typical cadmium sulfide photoconductor is shown in FIG. 1.

FIG. 1 illustrates the time dependence. of the photocurrent in a sample of cadmium sulfide when irradiated by beams of different photon energy. For example, when irradiated with a beam of photons of energy greater than about 2.2 electron volts, an enhancement of the photoconductivity is produced, as shown by the response curve A. For a beam of photons with energy less than about 1.6 electron volts, however, as shown by the response curve B, a clear-cut quenching of the photoconductivity is produced.

For many photoconductive materials, therefore, there is a particular photon energy which will cause quenching of the photoconductivity thereof. Further details of this quenching phenomenon may be had by reference to an 3 article entitled Note on Quenching of Photoconductivity in Cadmium Sulfide by E. A. Taft and M. H. Hebb in the Journal of the Optical Society in America, Vol. 42, No. 4, pages 24925 1, April 1952.

I have found that the response speed of an electroluminescent-photoconductor switching circuit, for example, may be greatly increased by applying an appropriate optical quenching signal to the photoconductor element at the very time that switching is desired. One embodiment of an electroluminescent switching circuit in accordance with my invention is shown in FIG. 2.

In FIG. 2 there is shown photoconductor element 1 disposed in light exchange relation with an electroluminescent element 2 and electrically in parallel circuit therewith. A selective optical feedback means 3, adapted to prevent optical feedback of photoconductivity stimulating light from the output of element 2 to element 1 and to allow such feedback of that portion of the output thereof capable of quenching the photoconductivity of photoconductor 1, is interposed between the elements 1 and 2. An appropriate voltage source 4 is connected across the circuit through a suitable impedance, shown conveniently as resistance 5.

As used throughout the specification and in the appended claims the term light is used in its broad sense as radiant energy and is not intended to be limited to only the visible spectrum. Further, the term photoconductivity stimulating light is intended to refer to radiation which increases the photoconductivity of the photoconductive material employed.

While my invention is not restricted to the use of particular photoconductive and electroluminescent materials, it will be understood that the specific elements for any given apparatus, circuit or system should be so selected to assure that at least a portion of the output of the electroluminescent element 2 is of a photon energy capable of quenching the photoconductivity of the photoconductor element 1 utilized therewith. Similarly, the selection of the type and magnitude of the voltage applied across the circuit is dependent upon the particular electroluminescent material selected. The selection of the particular voltage source, as well as the appropriate and most suitable optical feedback means, therefore, are determined to some extent by the selection of elements 1 and 2. Specific examples of materials for one typically suitable combination of elements I and 2 as well as the optical feedback means 3 are given hereinbelow with respect to the devices shown in FIGS. 3 and respectively to be presently described.

When the electroluminescent material selected for the element 2, for example, produces luminescence in response to an applied alternating current voltage, the voltage source 4 is selected accordingly to provide the appropriate electric field thereacross. It will be understood that when the electroluminescent material is capable of being excited to luminescence when a direct current voltage is connected thereto, such a selection is also appropriately made.

While my invention is subject to a wide range of applications it is particularly suited to the high speed and reliable detection of changes in the light level from a uniform background and its operation will be particularly described in this connection.

In the circuit arrangement of FIG. 2 the voltage across the parallel combination of elements 1 and 2 is adjusted so that with the incident light level from the uniform background at its steady state value, the output of light emitted by the electroluminescent means 2 is essentially zero. For example, the greater portion of the applied voltage appears across the impedance 5 and the voltage across the parallel combination of photoconductor I and electroluminescent means 2 is not sufiicient to excite the electroluminescent means to luminescence. That is, with incident light of the steady state value from the uniform background the impedance of the parallel branch is small 4, and the voltage appearing thereacross is insufficient to cause an electroluminescent output.

As the incident light level decreases, due to a defect in the surface of the uniform background, for example, the

resistance of the photoconductor element 1 increases, in-

creasing the impedance of the parallel circuit branch and causing a larger voltage to appear thereacross. This increased voltage causes a luminescent output to be produced from the electroluminescent element 2, which output contains at least some radiation having photons with energy to cause the photoconductivity of element 1 to be quenched. This portion of the output of electroluminescent element 2 is allowed to feed back through the selective optical feedback means 3. This further increases the resistance of the photoconductor element 1 thereby increasing the voltage across the element 2 and causing the output thereof to be increased. This action is cumulative and results in the element 2 rapidly producing a detectable luminescent output which may be detected in any convenient and well-known manner such as, for example, by a photoelectric cell or the like. The absorption by the photoconductor element 1 of photons of the particular energy allowed to feed back from the output of electroluminescent element 2 leads to the generation of the fast recombination centers to cause the photoconductivity thereof to be quenched.

When a photoconductor is not exposed to incident radi ation and a predetermined voltage is connected thereto, a given current known as the dark current will flow. When the surface of the photoconductor is illuminated by incident radiation the current flow will increase. This increase depends upon the intensity of the incident radiation and is known as the photocurrent. In many photoconductors the photocurrent is in the order of about 10 times the dark current. One convenient arrangement, therefore, in accordance with this invention is to select the components of the circuit arrangement shown in FIG. 2, for example, so that the impedance of the electroluminescent means 2 is approximately half way between the impedance of the photoconductor at its dark and photocurrent conditions.

In FIG. 3 there is shown one embodiment of an electroluminescent device in accordance with this invention. In FIG. 3 a photoconductive layer 6 of photoconductive material whose photoconductivity is quenched when subjected to irradiation of appropriate photon energy is disposed on one surface 7 of a substrate 8. Substrate 8 is of a material capable of rejecting radiation which increases the photoconductivity of the photoconductive material of layer 6 but passing radiation of the appropriate quenching photon energy therefor. A transparent conducting electrode 9, electroluminescent layer 10, and transparent conducting layer 11 are disposed, in order, on the opposite surface 12 of substrate 8. Electrolumines cent layer It interposed between transparent conducting electrodes 9 and II respectively provides an electroluminescent means indicated generally at 13 capable of being excited to luminescence by application of an appropriate electric field. Electroluminescent layer It) is of a material which provides an electroluminescent output wherein at least a portion is of the appropriate quenching photon energy for quenching the photoconduotivity of photoconductive layer 6.

A suitable photoconductive material for use as the photoconductive layer 6, for example, may be a typical cadmium sulfide material suitably activated to possess the property of varying its electrical conductivity under the influence of light. In FIG. 1 such a cadmium sulfide photoconductive material is shown, particularly by curve B, to exhibit a strong and very significant quenching of its photoconductivity when subjected to irradiation of photons of energy less than about 1.6 electron volts.

In FIG. 4 there is shown a graphical representation of the emission of 'one typical electroluminescent material suitable for use in accordance with this invention in com bination with a photoconductive material such as the cadmium sulfide described above whose photoconductivity characteristics are illustrated in FIG. 1. FIG. 4 shows the emission characteristic of an electroluminescent material of zinc sulfide activated with about 1.5 X 1O atoms of copper per cubic centimeter. As shown in FIG. 4 such a material has an appreciable electroluminescent output of photons with energies less than about 1.6 electron volts, which are the energies which cause a strong and significant quenching of the photoconductivity of the cadmium sulfide photoconductive layer 6.

For the combination of these specific materials for layers 6 and respectively, therefore, substrate 8 may be selected to reject radiation having photons with energies above about 1.6 electron volts but pass radiation having photons with energy less than about 1.6 electron volts. For example, substrate 8 may be an optical filter such as a Corning filter #7-69 manufactured by the Corning Glass Works, 1946 Crystal Street, Corning, New York. Various other materials are likewise suitable for substrate 8, the particular energy of the radiation to be rejected and passed depending upon the appropriate quenching energy of the photoconductive material of layer 6. Substrate 8, therefore, may be an appropriate optical filter such as in the example above, a semiconductive material having appropriate characteristics such as, for example, cadmium selenide, or other suitable materials.

The composite device illustrated in FIG. 3, therefore, comprises a substrate 8 having on one side thereof a photoconductive layer 6 whose photoconductivity is quenched when subjected to irradiation of appropriate photon energy and on the opposite surface thereof an electroluminescent means which when suitably excited to luminescence produces at least some output which is of photon energy capable of quenching the photoconductivity of the material of layer 6. The substrate 8 is of a material which allows optical feedback to photoconductive layer 6 of essentially only that portion of the electroluminescent output of electroluminescent means 13 which is capable of causing quenching of the photoconductivity thereof. Separate electrical connections 14, 15, 16 and 17 are provided to the ends of photoconductive layer 6 and to at least one end of transparent conducting electrodes 9 and 11 respectively. Electrodes 14, 15, 16 and 17 provide a means adapted to complete a parallel electrical circuit with the photoconductive layer 6 and electroluminescent means 13. For an application such as shown by the circuit in FIG. 2, for example, electrodes 14 and 16 may be suitably connected to one side of an appropriate voltage source while the electrodes and 17 may be connected to the other side thereof.

The operation of the device shown in FIG. 3 is the same as that described in detail for the apparatus of FIG. 2. For example, substrate 8 serves as the optical feedback means for allowing the optical feedback to photoconductive layer 6 of essentially only the portion of the electroluminescent output of electroluminescent means 13 which is capable of quenching the photoconductivity thereof.

In FIG. 5 there is shown a sectional view of an electroluminescent device in accordance with another embodiment of this invention. The device shown in FIG. 5 differs from that shown in FIG. 3 in that the substrate 8 is a body of high bulk resistivity semiconductive material, such as cadmium selenide for example, having the appropriate optical filtering characteristics for use with the material of photoconductive layer 6. A surface adjacent region 18 of semicondnctive substrate 8, remote from the layer 6, is of low resistance. A layer 10 of suitable electroluminescent material is disposed on this low resistance region 18 and transparent conducting material disposed on the other surface of layer 10 forms the conducting electrode 11; low resistance region 18 serving as the other conducting electrode 9. Separate connections 14, 15, 16 and 17 may be connected, as in the device of FIG. 3,

to the ends of the photoconductive layer 6 and at least one end of low resistance region 18 and the transparent conducting layer 11. Alternatively, the lower resistance region 18 may be conveniently extended to include the surface adjacent region of the end portion of semiconduotive substrate 8, as shown in FIG. 5, so that a layer of conducting material 19 may be employed to suitably join one end of photoconductor layer 6 with this low resistance region so that the single electrode 15 may be conveniently utilized as a single connection to one end of photoconductive layer 6 and low resistance region 18. As before, electrodes 14, 15 and 17 thus provide a means adapted to complete a parallel electrical circuit with photoconductive layer 6 and electroluminescent layer 10.

There has been described hereinbefore electroluminescent devices and circuits utilizing a new arrangement of electroluminescent and photoconductor elements which provides of greatly increased response speed and sensitivity by applying an optical quenching signal to the photoconductor element. The optical quenching signal so applied increases the response speed of the apparatus, system, or circuit by increasing the rate of decay of the photocurrent in the photoconductor element. This decrease in the decay time greatly reduces the time required for the electroluminescent element to read full output after the incident light level has decreased below a certain threshold value. As described hereinbefore there is a regenerative quenching action since the feed-back of even a small intensity quenching signal causes an increase in the voltage across the electroluminescent means resulting in an increased output and a higher intensity quenching signal being fed back to the photoconductor to still further quench its photoconductivity and increase the electroluminescent output, and so on.

While only certain preferred features of my invention have been shown by way of illustration, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A light sensitive detector comprising: a substrate having on one surface thereof a layer of photoconductive material the photoconductivity of which is quenched by radiation of appropriate photon energy value and on the opposite surface thereof an electroluminescent means only a portion of whose electroluminescent output is of photon energy value capable of quenching the photoconductivity of said photoconductive layer, said substrate being essentially impervious to radiation other than that corresponding to said appropriate quenching photon energy; and means adapted to complete a parallel electrical circuit with said photoconductor and said electroluminescent means.

2. A light sensitive detector comprising: a substrate having on one surface thereof a layer of photoconductive material the photoconductivity of which is quenched by radiation of appropriate photon energy value and on the opposite surface thereof an electroluminescent means only a portion of whose output is of photon energy value cap able of quenching the photoconductivity of said photoconductive material, said substrate being essentially impervious to radiation which increases the photoconductivity of said photoconductive material and pervious to radiation of photon energy of said appropriate quenching value; and means adapted to complete a parallel electrical circuit with said photoconductor and said electroluminescent means.

3. An electroluminescent device comprising in order: a layer of photoconductive material whose photoconductivity is quenched by photons of appropriate energy values; a layer of high bulk resistivity semiconductive material having a low resistance surface adjacent region remote from said photoconductive layer, said layer of semiconductive material optically passing essentially only photons having energies which quench the photoconductivity of said photoconductive material; a layer of electroluminescent material having only a portion of its output with said appropriate quenching energy; and means adapted to complete a parallel electrical circuit with said photoconductor layer and said electroluminescent layer.

4. An electroluminescent device comprising in order: a layer of photoconductive material exhibiting a quenching of its photoconductivity when subjected to irradiation of appropriate photon energy value; a layer of high bulk resistivity semiconductive material having a low resistance surface adjacent region on at least one end and on the major surface opposite said photoconductive layer, said layer of semiconductive material being selected to optically pass essentially only photons having energies which quench the photoconductivity of said photoconductive layer; a layer of electroluminescent material capable of being excited to luminescence by an appropriate electric field, only a portion of the output of said electroluminescen t layer being photons with said appropriate quenching energy values; and means adapted to complete a parallel electrical circuit With said photoconduetor layer and said electroluminescent layer.

References Cited by the Examiner UNITED STATES PATENTS 2,773,992 12/56 Villery 250--213 2,870,342 1/59 Walker et al. 2502l3 2,883,556 4/59 Jenney et al. 2502l3 2,890,350 6/59 Lempicki 250-213 2,891,169 6/59 Nicoll 250-213 2,896,087 7/59 Kazan 2502l3 2,896,088 7/59 Lempert 2502l3 2,904,697 9/59 Halsted 250-2l3 2,909,667 10/59 Orthuber et al. 250-2'l3 2,957,991 10/60 Kazan 25021 3 3,039,005 6/62 OConnell et al. 2502l3 RALPH G. NILSON, Primary Examiner.

ARCHIE R. BORCHELT, Examiner. 

1. A LIGHT SENSITIVE DETECTOR COMPRISING: A SUBSTRATE HAVING ON ONE SURFACE THEREOF A LAYER OF PHOTOCONDUCTIVE MATERIAL THE PHOTOCONDUCTIVITY OF WHICH IS QUENCHED BY RADIATION OF APPROPRIATE PHOTON ENERGY VALUE AND ON THE OPPOSITE SURFACE THEREOF AN ELECTROLUMINESCENT MEANS ONLY A PORTION OF WHOSE ELECTROLUMINESCENT OUTPUT IS OF PHOTON ENERGY VALUE CAPABLE OF QUENCHING THE PHOTOCONDUCTIVITY OF SAID PHOTOCONDUCTIVE LAYER, SAID SUBSTRATE BEING ESSENTIALLY IMPERVIOUS TO RADIATION OTHER THAN THAT CORRESPONDINT TO SAID APPROPRIATE QUENCHING PHOTON ENERGY; AND MEANS ADAPTED TO COMPLETE A PARALLEL ELECTRICAL CIRCUIT WITH SAID PHOTOCONDUCTOR AND SAID ELECTROLUMINESCENT MEANS. 